In making a thin film transistor (“TFT”) used in a display device, a semiconductor layer is formed such that an amorphous silicon layer is deposited on a transparent substrate made of glass or quartz and the amorphous silicon layer is subjected to a dehydrogenation treatment and is crystallized.
At this time, the semiconductor layer which constitutes a source, a drain and a channel area is formed by depositing an amorphous silicon layer on a transparent substrate made of a material such as glass using a chemical vapor deposition (“CVD”) technique. The silicon layer deposited directly on the substrate using a CVD technique is an amorphous silicon layer which contains about 12% of hydrogen and thus has a low electron mobility, and when the amorphous silicon layer having such a low electron mobility is heat-treated and crystallized into a silicon layer of crystalloid structure having a high electron mobility, the silicon layer may be damaged since the hydrogen contained therein may explode. In order to prevent such a hydrogen explosion from occurring during crystallization, a dehydrogenation process is carried out. The dehydrogenation process is performed such that a heat-treatment step is performed in the furnace at a temperature of more than about 400° C. for tens of minutes to tens of hours. Then, the dehydrogenated amorphous silicon layer is subjected to a crystallization process.
The crystallization technique which crystallizes an amorphous silicon layer to form a polysilicon layer includes a solid phase crystallization technique and an excimer laser crystallization technique. The solid phase crystallization technique is one which heat-treats and crystallizes an amorphous silicon layer for several hours to tens of hours at a temperature less than about 700° C. which is a temperature that may transform the glass which forms the substrate of a display device on which the TFT is formed. The excimer laser crystallization process is one which scans an excimer laser to an amorphous silicon layer to be heated and crystallized at a high temperature for a very short time.
However, the solid phase crystallization technique has disadvantages in that a relatively lengthy processing time is required and the substrate is exposed to a high temperature for a long time and thus, the glass of the substrate may transform. The excimer laser crystallization technique also has disadvantages in that a high-priced laser device is needed and also, an extrusion may occur on a crystallized surface forming an undesirable interface characteristic between a semiconductor layer and a gate insulating layer.